1. Field of the Invention
The present invention relates to image processing for correcting an image read from an original document.
2. Description of the Related Art
[Fading Correction]
With improvement of performance, low cost, and simpler manipulation of a printer, scanner, and multi-functional peripheral equipment incorporating the functions of a printer and scanner, printing, scanning, copying, and the like have become easy. In addition, although documents and simple diagrams have been scanned, printed, or copied conventionally, photographs are also scanned, printed, or copied in these days. Since user's demand for the image quality of a photograph is high and the color reproducibility of print or copy is easy to recognize for users, color reproducibility is an important factor that determines image quality.
When an image of a silver halide film, photograph print, or printed material is exposed to light or air, it fades over time. In order to improve the resistance of a film, photograph print, or printed material, color materials and printing media have been improved. However, it is difficult to restore the colors of the image of an already faded film, photograph print, or printed material.
To solve this problem, a technique of acquiring the feature amount of an image or information for correction to perform fading correction of the image is available. For example, one technique (first technique) analyzes the density information of the image data of an original document, generates information (reproduction information) about an image reproduction condition, and adds it to the image data. When printing the image data with the reproduction information added thereto, the image data is corrected based on the reproduction condition to reproduce a color close to the image of the original document.
Another technique (second technique) reads an image of an original document and analyzes the read image to calculate image information. The calculated image information is printed outside an image formation area upon printing the read image. When the above-described image information is detected upon reading the image of the original document, the read image undergoes image processing based on that image information, and the processed image is printed.
According to these techniques, it is possible to correct an image read from a faded image of an original document to print an image close to the original color of the image of the original document.
[Problem about Color Reproduction]
The color of an image of an original document and the color of an image acquired by copying the image of the original document change due to differences in the color reproduction characteristics of devices.
In addition, there is also a problem such as metamerism in which color differences occur due to the difference between the RGB sensitivity of human vision and the RGB sensitivity of the sensor of a scanner. The color of an object recognized by a receptor such as an eye or sensor is determined according to the incident light, the reflection spectrum of the object, and the respective R, G, and B color sensitivities of the receptor. For this reason, even when the incident light and the reflection spectrum of an object remain the same, receptors with different sensitivities recognize different colors. Conversely, even when the incident light and the reflection spectrum of an object change, a receptor may recognize the same color depending on the distribution of its sensitivity.
For example, since a silver halide photograph print and a printed material of an inkjet print use different color materials and their image surface structures are also different, their reflection spectra are different. For this reason, colors perceived as the same color by the human vision are recognized as different colors by the sensor of a scanner having a sensitivity different from that of the vision.
In order to solve this problem, a technique of identifying the printing method of an image of an original document and modifying an image signal read from the image of the original document by using a color modification parameter corresponding to the identified printing method is available. This technique requires preparation of color modification parameters corresponding to various printing methods in advance. However, since the varieties of printing apparatuses and printing media continue to increase, it is difficult to generate color modification parameters for all of them.
[Problem of Color Correction]
The first and second techniques perform color correction using information which represents color balance, such as the RGB signal value and CMYK signal value of a solid color portion or skin color portion, the average value of luminances or color components, or the like of an image of an original image. In recent scanners and printers, however, satisfactory color correction cannot be done by the first and second techniques. The reason for this will be described next.
First, a color reproduced by a device does not always become the same color for the same RGB data. More specifically, in the case of a printer, a color gamut is determined according to a color material, printing medium; and the like, and therefore a color that falls outside the color gamut cannot be reproduced. For this reason, a printer compresses the color gamut of image data by performing gamut mapping. Compression of a color gamut differs depending on an intent for colorimetric matching, a high tonality level, or the like. As a result, even when the same RGB data as the original image is restored, the colors of the original image cannot always be reproduced due to the influences of a printer, print mode, color material, printing media, and the like.
Also, there is a problem notable in an inkjet printing method of a serial scan type in that a reproduced color changes depending of a printing method even when the amount of ink remains the same.
In an inkjet printing method of a serial scan type, an ink discharge unit discharges ink while reciprocating on a printing medium. During this operation, another operation (paper feed) to feed the printing medium in a direction perpendicular to the reciprocating movement of the ink discharge unit is also performed, so that ink can be eventually applied to the entire area of the printing medium. In such a method, the following problems arise.
First, a color printed in the forward movement of the ink discharge unit is sometimes different from that printed in the backward movement even when the amount of ink remains the same. For example, when printing blue, a cyan C ink and magenta M ink are used. In a forward movement shown in FIG. 1A in which an ink discharge unit 70 moves in a direction F, inks are discharged in the order of C to M at a single position on a printing medium 71. On the contrary, in a backward movement shown in FIG. 1B in which the ink discharge unit 70 moves in a direction B, inks are discharged in the order of M to C at a single position on the printing medium 71. In case of ink with a relatively high permeability that is often used by an inkjet printing apparatus, a phenomenon occurs in which an ink discharged at a later time gets under an ink discharged at an earlier time. As a result, the color of the ink discharged first becomes dominant. More specifically, when inks are discharged in the order of C to M, blue close to cyan appears, and when inks are discharged in the order of M to C, blue close to magenta appears. Accordingly, even when an amount of ink remains the same, different colors are reproduced in the forward and backward movements.
In addition, in an inkjet printing method, a phenomenon is known in which an ink droplet splits into a plurality of droplets upon one ink discharge. The plurality of split droplets reach a printing medium with a time difference, but their landing positions also shift along with the movement of the ink discharge unit 70. When the landing positions shift, the overlying manner of the inks on the printing medium also changes, so that an area factor changes and influences color reproduction as differences in density. In addition, a shift in the landing position in the forward movement and that in the landing position in the backward movement are different from each other in shift amount and position. Therefore, even when one kind of ink is discharged in the forward and backward movements, density or color may change due to the shift in the landing position.
Furthermore, the density or color changes due to the mechanical deviation of a paper feed mechanism or the driving mechanism of the ink discharge unit 70. More specifically, due to a mechanical deviation, landing positions on a printing medium vary, resulting in change in density or color. Particularly, the upper and lower end portions of a printing medium are known as positions where a landing position easily shifts since the printing medium cannot be sufficiently held.
As described above, in an inkjet printing method, even when ink of the same amount is discharged for a given pixel, the color of the pixel reproduced on a printing medium does not always become uniform due to influences such as the moving direction of the ink discharge unit 70, ink discharge status, mechanical deviation, and the like, and therefore each color changes depending on positions.
Against these problems, various countermeasures have been employed such as keeping a constant scan direction of an ink discharge unit, keeping a constant ink discharge order by preparing a plurality of ink discharge units, employing a multipath printing method in which an ink discharge unit scans a plurality of times on a printing medium, using a precision driving mechanism, and the like. However, these countermeasures lead to drop in printing speed or increase in cost, and it is difficult to balance it with improvement in image quality.
[Problem of Fading Correction]
Fading is a phenomenon that occurs when a color material is damaged by light, heat, oxygen in air, or the like. Some color materials are strong against damage, and other color materials are weak against damage. Accordingly, not all colors uniformly fade. For example, assume that a cyan ink has a high fading resistance and a magenta ink has a low fading resistance. In this case, even if they are balanced at the initial stage of printing, as fading advances, magenta becomes pale and cyan becomes a relatively dominant color.
When only color materials of cyan C, magenta M, and yellow Y are used, simple processing can be performed as color processing, which is expressed by:C=255−R M=255−G Y=255−B  (1)
In this case, when the fading amount of each of C, M, and Y is known, it is relatively easy to reproduce the original color of an image from a signal value acquired by reading the faded image.
Nowadays, however, in order to improve image quality, use of black K or gray Gy, a homochromatic light color, colors with different hues (e.g., red R and green G) has become common in addition to basic C, M, and Y colors. As the number of ink types increases, the progression degree of fading is complicated.
In addition, in order to correct ink characteristics, not a simple method expressed by equation (1) but a complicated process using a matrix operation or lookup table (LUT) is employed for color conversion processing. In other words, a color separation process for determining a combination of ink amounts for RGB data is significantly complicated.
Accordingly, when conventional fading correction based on the average value of the three primary colors such as RGB or CMY or the average value of the color components over an entire image analyzed from a histogram is performed, correction is insufficient in some colors and excessive correction is performed in some colors. For example, assume that the image of an original document in which a magenta hue has faded little and a red hue has greatly faded is corrected based on the RGB average values or a histogram. In this case, when a correction amount is determined according to the red hue, an R component correction amount becomes large. However, since the magenta hue has faded little, when the R component correction amount according to the red hue is applied, excessive correction is performed in the magenta hue. Of course, when a correction amount is determined according to the magenta hue, correction in the red hue is insufficient.
As described above, it is difficult to correct differences in recognition of a color by a receptor and fading with a high performance by using information representing color balance such as the RGB signal values or CMYK signal values in a solid portion and skin color portion, the average value of luminances or color components, or the like of the image of an original document, information representing the type of an original document, and the like.
[Other Problems]
There are other cases in which it is preferable to correct the read image of an original document upon copying or scanning, in addition to a case of fading of the image of the original document. For example, when the color reproduction range of the original document is narrow or when color heterogeneity has occurred due to a problem of a printer used to print the original document, it is preferable to correct the read image of the original document.
An MFP adopting an inkjet printing method is compatible with various types of printing media such as a plain paper sheet or photograph printing sheet. The quality of a printed image greatly changes depending on a printing medium. That is, a color gamut reproducible by a printing medium is one of factors that determine the quality of a printed image. When a printing medium with a wide reproducible color gamut such as a photograph printing sheet is used, a vivid color can be vividly reproduced. On the other hand, when a printing medium with a narrow reproducible color gamut such as a plan paper sheet is used, a vivid color is inevitably reproduced as a color with a low saturation.
On the contrary, when an image with a narrow color gamut is printed on a printing medium with a wide reproducible color gamut, it can be discriminated that the color gamut of an image read from the printed image is narrow. However, it cannot be distinguished whether the narrow color gamut is due to the narrow color gamut of the original image or it becomes narrow when printing the image of the original document. Accordingly, the read image with the narrow color gamut will be printed without any correction.
FIG. 2 is a view schematically showing the color gamuts of images. For example, assume that an original image is a photograph printed on a photograph printing sheet, and a copy image acquired by copying the original image is printed on a plain paper sheet. In addition, assume that a second generation copy image acquired by copying the copy image is printed on a photograph printing sheet.
The photograph printing sheet can reproduce a wide color gamut 80. On the other hand, the plain paper sheet can reproduce a color gamut 81 narrower than the color gamut 80. When the original image has a wider color gamut 82 than the color gamut 81 of the plain paper sheet, gamut mapping is performed upon copying the image onto the plain paper sheet, and therefore the color gamut of the image is compressed into the color gamut 81 or a further narrower color gamut 83. That is, image conversion such as decreasing saturation or the like is performed by gamut mapping.
When the copy image of the plain paper sheet is copied onto the photograph printing sheet to produce a second generation copy image, since the color gamut of the copy image is the color gamut 83, the second generation copy image has a color gamut 84 similar to the color gamut 83. That is, although the photograph printing sheet capable of reproducing the color gamut 80 is used, its capability is not made full use, and the second generation copy image results in an image with a decreased saturation.
Also, color heterogeneity occurs due to the above-described mechanical deviation or the like in some printers upon printing an original image. Generally, when there is color heterogeneity, it is advantageous for a user to copy an image after correcting the color heterogeneity.